Piston



March 31, 1942. w, o uc 2,278,149

PISTON Filed Nov. 12, 1941 1164.1. IIG.2.

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A1. FEED W EOEBUCIC Patented Mar. 31, 1942 UNITED STATES 1 PATENT OFFICE V 2,278,149 I PISTON Alfred Waltcr Roebuck, Radlett, England Application November 12, 1941, SerialNo. 418,809

In Great Britain December 5, 1939 1 Claim. (01. 309 14} This invention relates to pistons and concerns composite pistons which comprise a body made of one substance and a separate shell made of another substance, said substances having different co-efficients of expansion. In this connection, the invention is particularly concerned with pistons, comprising a lightweight non-ferrous body, for instance made of aluminium or an alloy thereof and a ferrous shell such as one of cast iron, and which seek to retain the recognised advantages inherent to these different materials whilst obviating their disadvantages. Difficulty generally has been experienced hitherto in allowing for the different co-eflicients of expansion of the metals employed in such a way as to avoid the possibility of distortion whilst at the same time guarding against loose interfitting of the components. Also, in some cases, it has been necessary to adhere to machining limits that are not generally regarded as being commercial.

Objects of the present invention are to obviate the above difficulties and to construct a composite piston in such a manner as to aim at efficient operation over long period of use, and

ably locked or fixed together at one position and are relatively formed to leave at a remote position an expansion clearance between co-ordinated abutments of the body and shell, and that spring or like resilient means is provided in the said expansion clearance, the resiliency of said spring means accommodating the difference of expansion of the two parts. I

In a preferred form of the invention the piston comprises a body made of aluminium or an alloy thereof and which is formed with a peripheral recess in its lower part, and a separate ferrous shell constituting a skirt which is accommodated by said recess, means provided at or near one end of the shell for locking or fixing said parts together so as to be relatively immovable in endwise direction, co-ordinated abutments to the body and shell which are spaced apart to leave an expansion clearance at or near the other end of the shell, and spring or like resilient means occupying said expansion clearance, the resiliency of said spring means accommodating the difference of expansion of the two parts. The

locking or fixing means is preferably arranged at or near'the bottom end of the piston, and in this case the co-ordinatedi abutments comprise the upper end face of the shell and a shoulderon' the body disposed, beneath a, -c0mpression-ringcarrying, head portion thereof.

' In the accompanying drawing which illustrates a piston constructed according to a practical embodiment of the invention,

Figure 1 is a longitudinal section through the piston in they plane :of the gudgeon pinbosses,

Figure 2 is a longitudinal section at 1 right angles to the plane of Figure 1,

Figure 3 is an underneath plan. view,

Figure his an elevation of the, piston, and

- Figures 5, and. 6 are fragmentary perspective views of. the lower parts of the piston body and shell,'il:lustrating the interlock means.

' Referring to the drawing, the piston shown comprises a lightweightzhollow body 1 preferably made of aluminium or an alloy. thereof, said body having piston ring. grooves 8 formed in the peripheryl of itsupper part or head. The lower or major part of said body is made of reduced diameter to provide a peripheral recess, such reduced diameter portion extending up to the ring-carrying head and down to the bottom extremity of said body. Gaps 9 are formed in the bottom of the body, said gaps 9 being equally spaced circumferentially, and peripheral grooves III are formed in the feet H which remain between the gaps. The gaps 9 may be out only part way into the grooves l0 (Figure 5), in which case there is a single groove only, having a continuous upper wall and base.

The second component consists of a cylindrical shell I2 of ferrous metal, preferably cast iron, the external diameter of which corresponds to or is slightly greater than the diameter of the body head, the said shell l2 being adapted to fit on the reduced lower or main portion of the body. Said shell I2 is formed on its inner surface with an interrupted annular rib, the rib portions 83 being adapted, when sliding the shell on to the body, to pass up into the gaps 9 formed in the body, and after thus sliding the body as far as required, to enter the grooves II] in the foot portions ll of the body on imparting turning movement to'the shell relative to the body, thus providing a bayonet joint. The relative widths of said rib portions l3 and grooves H! are such as to allow slight vertical clearance for said rib portions.

The shell 12 is of such a length above the ribs l3 thereof as not to extend the full distance up to the body shoulder I4 at the top of the reduced portion but to leave a narrow intermediate groove I5. In the said groove I there is fitted set screw I! or a rivet may be fixed in aligning holes in said components, said screw or rivet passing through one of the rib portions I3 as shown or being otherwise arranged.

As will be readily understood, the components of the piston are positively interlocked by the ribs I3 and grooves ID to render said components relatively immovable in 'endwise direction, that is to say to prevent downward sliding movement of the shell I2 relative to the body 1. At the same time the freedom which the ribs I3 have within the grooves I0 and the clearance I5 between the upper edge of th shell and the shoulder I4 of the body, allow for expansion or relative expansion of the ribs and shell itself, the spring ring I6 or equivalent device expanding laterally to permit the greater longitudinal expansion of the body whilst maintaining firm interfitting of the two components at all times. Further, the employment of the said spring ring IE5 or equivalent avoids the necessity for observing close machining limits at those points where the shell and body interlock vertically.

As shown, each of the foot portions II of the body member, and each of the intermediate gaps 9 subtends an arc of approximately 90 degrees, the centres of the gaps 9 being vertically aligned with the gudgeon pin bosses I8. This provides the foot portions I I atthe positions where lateral thrust is exerted, ample provision being thus afforded for taking that thrust. The depending wall of the body lying beneath the ringgrooved head may be cut away or formed with openings I9 for lightness in weight, the cutaway portions or openings being vertically above the foot portions II of the body and being of such a size and shape as not to impair the required strength or the body member. In a zone intermediate in its height between the grooves l0 and shoulder I4, the body may be more deeply recessed, i. e. throughout the dimension a (Figure 2) to afford a space 20 between said body and the shell. For the same purpose the shell I2 may have an upper rim or seating portion 2| of greater thickness than the remainder of said shell. If necessary, in order to accommodate a spring ring of satisfactory width, a groove 22 may be formed in the body member to coincide with and increase the depth of the space afiorded between the top of the shell and the shoulder of the body member.

The method of construction hereinbeiore set forth permits of the shell being made by the centrifugal casting process, the shell when thus made possessing the recognized advantage inherent to metal cast in that manner. In this case, an annular internal rib can be formed during the said process and be subsequently interrupted by a simple machining operation to form the separate rib portions I3 aforesaid.

Although for the sake of clearness the spring ring I6 is shown in an appreciably expanded condition, said spring in practice will normally have a substantially flatter condition, that is to say when the parts are cold.

I claim:

A piston comprising, in combination, a body having ring grooves at one end with extensions provided with gudgeon pin bosses and a medially located peripheral recess, the bottom edge of said body extensions being formed with gaps disposed to leave foot portions positioned at right angles to said gudgeon pin bosses, said foot portions having grooves formed therein and extending laterally from the said gaps, and a shell having a different coeificent of expansion from the body and surrounding thesame to constitute a piston skirt whose upper edge terminates within the limits of said medial recess, ribs formed internally on the shell and loosely fitted into the grooves through the gaps to hold the body and shell interengaged against substantial movement in an axial direction, and spring means abutting one wall of said medial recess between said ribs and grooves.

ALFRED WALTER ROEBUCK. 

